[Router] Add load-aware prefill admission and bounded policy proposals (#37843)

Signed-off-by: Vincent Gao <vincentbo@linux.alibaba.com>
Co-authored-by: Kangyan Zhou <zky314343421@gmail.com>
Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Co-authored-by: Shangming Cai <csmthu@gmail.com>

experimental/sgl-router/tests/proxy/main.rs

@@ -19,6 +19,7 @@ mod header_forwarding;
 mod pd_bootstrap_injection;
 mod pd_pool_isolation;
 mod roundrobin_input_ids;
+mod shared_prefill_admission;
 mod sticky_input_ids;
 mod sticky_routing;
 mod timeout;